Conventional inkjet printers typically operate by ejecting small droplets of ink from individual orifices in an array of such orifices provided on a nozzle plate of a printhead. The printhead may form part of a print cartridge which can be moved relative to a sheet of paper and the timed ejection of droplets from particular orifices as the printhead and paper are relatively moved enables characters, images and other graphical material to be printed on the paper.
A simplified plan view of a typical conventional printhead is shown in FIG. 1. It is fabricated on a silicon substrate 10 having thin film resistors 12 and associated thin film circuitry (not shown) deposited on its front surface (i.e. the surface facing the viewer in FIG. 1). The resistors 12 are arranged in an array relative to one or more ink supply slots 14 in the substrate, and a barrier material 16 is formed on the substrate around the resistors to isolate each resistor inside a respective thermal ejection chamber 18. The barrier material 16 is shaped both to form the thermal ejection chambers 18 and to provide an ink communication channel 20 between each chamber 18 and the ink supply slot 14. In this way, the thermal ejection chambers 18 are filled by capillary action with ink from the ink supply slot 14, which itself is supplied with ink from an ink reservoir in the print cartridge of which the printhead forms part.
The composite assembly described above is typically capped by a nozzle plate, for example of nickel or polyimide, which is not shown in FIG. 1 to avoid obscuring the underlying detail. The nozzle plate has an array of orifices which correspond to and overlie the ejection chambers 18 so that each orifice is in register with a respective resistor 12. The printhead is thus sealed by the nozzle plate, but permits ink flow from the print cartridge via the orifices in the nozzle plate.
The printhead operates under the control of printer control circuitry which is configured to energise individual resistors according to the desired pattern to be printed. When a resistor is energised it quickly heats up and superheats a small amount of the adjacent ink in the thermal ejection chamber. The superheated volume of ink expands due to explosive evaporation and this causes a droplet of ink above the expanding superheated ink to be ejected from the chamber via the associated orifice in the nozzle plate.
FIG. 1 shows a printhead where a series of thin film heating resistors 12, and corresponding nozzles, are disposed along each side of a single ink supply slot 14. However, many variations on this basic construction will be well known to the skilled person. For example, a number of arrays of orifices and chambers may be provided on a given printhead, each array being in communication with a different coloured ink reservoir. The configurations of the ink supply slots, thin film circuitry, barrier material and nozzle plate are open to many variations, as are the materials from which they are made and the manner of their manufacture.
The typical printhead described above is normally manufactured simultaneously with many similar such printheads on a large area silicon wafer which is only divided up into individual printhead dies at a late stage in the manufacture.
Existing printhead technology is not suitable for newly-emerging industrial applications in which it is desired to print using “ink” comprising suspensions of, for example, ceramic particles in strong solvents and acid bases. Thus, printheads made using photoresist as the barrier material are not resistant to chemicals such as acids, bases, etc. or the presence of solvents such as toluene, and tend to delaminate from the die or the nozzle plate and fail soon after operation. Printheads made using a polyimide orifice plate are not durable to the jetting of ceramic materials as these hard particle will cause rapid wear in the soft nozzle material resulting in continuously increasing drop weight and increases in drop misdirection. Soft nozzle materials are also prone to scratching in use, another cause of misdirection.
There is therefore an emerging needs for industrial print heads that are resistant to attack from acids/alkalis/solvents and that have good mechanical abrasion/wear resistance to allow thermal inkjets to be used for new applications such as the precise deposition of functional materials, e.g. liquids intended to form conductors and resistors in miniature electrical circuits.
It is an object of the invention to provide an improved method of making an inkjet printhead in which, at least in certain embodiments, these needs are met.